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186                         METALLURGY OF IRON" AND STEEL.
as 1 per cent, of silicon if blown sufficiently hot, but in the open hearth there is no chance for the bath to arrive at an intense degree of heat as long as a considerable percentage of this element is present; for superheating is not readily attained without a lively bath, and the bath will very seldom be lively as long as it holds a high content of silicon. Thus the open hearth cannot rival the converter in producing high-silicon metal by non-combustion, but under suitable conditions the amount carried along in the metal may be quite appreciable, and, by holding the bath at a very high temperature with a silicious slag, there will even be a reduction of the silica of the hearth. This variation in affinity plays an important part in the production of steel castings.
The presence of silicon, due to high temperature, tends to prevent the absorption of gases, and it is stated by Odelstjerna* that if at any time the metal is allowed to cool, so that the last traces of silicon are burned, the gases which are absorbed cannot be expelled by a subsequent superheating.
Odelstjerna is doubtless correct in his statements, but there may be other factors involved in a full explanation. It is certain that in the manufacture of small ingots to be rolled directly into plates, there are delicate adjustments of temperature and slag that are not easily explained by considering silicon alone. One of these factors is the extent and force of the oxidizing influence. It is the opinion of some metallurgists that the best quality of open-hearth steel can only be made when the burning of the metalloids is carried on at a slow rate, so that the bath shall not contain an excess of oxygen at any time, and it is stated by Ehrenwertht that a certain American works makes a practice of keeping a charge in the furnace a very long time when a good quality of steel is desired. As a matter of fact, the works in question did carry out such a system at one time out of respect to foreign tradition, but found no advantage in so doing, and has discontinued the practice.
It is also an opinion, held by men of reputation, that a high proportion of pig-iron in the original charge will give a superior product. If this is true, it probably arises from the fact that the presence of a high proportion of carbon after melting, with the
* Trans. A. I. M. JB., Vol. XXIV, p. 808.
t Das Berg- und Hiiltenwesen auf der Weltausstellung t*t Chicago.   Ehrenwerth, 1895, P. 276.